It is known within the field of indirect heat exchangers that the transfer of heat from a first to a second heat exchanging fluid is a combined function of the relative masses, differential temperature, and specific heats of the two fluids in comparison to the thickness and mass of the intervening heat exchange structure which separates the two fluids.
When the fluids within a heat exchanger are of relatively low specific heat or differential temperature, the efficiency of the heat exchanger of the indirect type is often adversely affected by the thickness of the heat exchange walls. Minimum wall thickness is established by manufacturing requirements necessary to obtain a realizable heat exchanger. In compensation for resulting resistance to heat flow, various techniques have been used to lengthen the flow path and the flow time for both fluids in the heat exchanger so as to provide maximal heating transfer. Thus an indirect heat exchanger is often seen to include many elongated fins, zig-zags, or other difficult to manufacture impediments to fluid flow, all of which are intended to lengthen the contact time of the fluid with the heat exchanger and thus increase the heat transfer per unit of fluid passed.
The most common method of forming such heat exchangers is by soldering or welding together multiple, corrugated, thin wall metallic structures. Since the structure must be fluid tight, manufacturing tolerances are extremely critical, and extensive effort is required to insure that leak tight soldered joints or welded structures have been formed. In addition, the resulting structure, for low specific heat fluids, can become extremely large and expensive to manufacture.
More recently, certain heat exchange structures, especially within the area of rocket engines, have been developed by the process of forming one wall of the heat exchanger structure, casting or attaching a meltable material to that wall, carving the meltable material so as to create a desired surface of a second wall of the heat exchange structure, electroforming a second wall upon the substrate, and then removing the substrate by melting. While this process eliminates many soldering and welding steps, and thus decreases one particular failure area, it requires individual, often manual, one time creation to form the substrate, since the defining substrate is destroyed in the process of creating the final product. It requires the formation of a first heat exchanger wall as a separate task. This technique is thus seen primarily in low production rate, high precision products such as rocket engine combustor thrust nozzles.